1. Field of the Invention
The present invention relates to a temperature adjustment device installed with a heat generating element, to a laser module, and more particularly, to a temperature adjustment device such as, for example, a thermoelectric element (peltier element) that requires temperature adjustment in a wide range such as a wavelength variable laser diode (LD), and to a laser module provided with the thermoelectric element.
2. Related Art
In a Dense Wavelength Division Multiplexing, required for a laser diode (LD) used as a signal light source are (i) high spectral purity and (ii) a wavelength of the light source which does not vary with ambient temperature, etc. Meanwhile, required for a laser diode (LD) used as a wavelength variable signal light source is enabling a wavelength of the light source to be varied in some range, as well as satisfying the above-mentioned conditions (i) and (ii). In order to achieve the above-mentioned condition (i), considered as a structure of LD are DFB (Distributed Feedback-Laser Diode), EBR-LD (Distribute-Bragg Reflector LD), etc. In order to achieve the above-mentioned condition (ii), various methods have been proposed, and a method is in practical use of controlling temperature of LD to control the wavelength.
Currently, in DWDM, a single printed circuit board installed with a single signal light source laser module and accompanying electric circuitry forms a signal light source of a single wave. In other words, DWDM of 256 waves is constructed of 256 printed circuit boards, and the printed circuit boards and accompanying electrical equipment is stored in a single cabinet. Power consumption of a single printed circuit board is about 20 W, a plurality of printed circuit boards and accompanying electrical equipment is stored in a small-sized cabinet, and therefore, the entire amount of generated heat is considerably large. Accordingly, inside the housing of the signal light source laser module is exposed to high temperature, and generally, operation reliability at ambient temperatures of 70° or more is required of the signal light source laser module.
To clarify problems of the conventional signal light source laser module, following descriptions are given of a wavelength variable signal light source laser module under a condition with ambient temperature of 70° C. In the variable wavelength light source, in order to obtain a desired wavelength, a laser diode is disposed on a temperature adjustment device and the temperature is controlled. Generally, the temperature of the laser diode is controlled in a range of 50° C. to 0° C. As an example, a case will be described below where the temperature of LD is controlled to 0° C. . FIG. 11 is a cross sectional view of the conventional laser module.
In FIG. 11, it is assumed that the temperature inside the housing of the signal light source laser module is 70° C. , and that the temperature of the laser diode is 0° C. In the conventional signal light source laser module 100, in order to prevent an axis shift from occurring in an optical system 102 due to variations in temperature, a laser diode 101 and a laser carrier 104 are mounted on an upper plate of a thermoelectric cooling module 103 through a thermal conductive plate 108. In other words, when the temperature of the laser diode 101 is 0° C., the optical system 102 and laser carrier 104 are also cooled to 0° C. or around 0° C. Meanwhile, a spontaneous heat flow develops in a portion with low temperature cooled as described above from the housing 107.
The heat flow develops through gas molecules as media sealed in the laser module 100, radiation heat transfer and heat conduction via wire that supplies power to the laser diode. For example, in the housing with an inner area of 10 mm×20 mm×6 mm, the heat flow becomes about several hundred mW to 1 W. The thermoelectric cooling module needs power so as to transfer heat Q flowing to an upper substrate of the thermoelectric cooling module and heat of the laser diode to a high-temperature substrate of the thermoelectric cooling module. In addition, in the signal light source such as a variable wavelength light source, since about 20 mW is required as a light output, the amount of generated heat of the laser diode is 0.1 W at most.
Ideally, it is desired that the thermoelectric cooling module maintains the temperature of the laser diode at 0° C. by supply of power only for transferring the heat (0.1 W at most) of the laser diode. However, the module actually consumes much power because the module needs to transfer the generated heat of the laser diode and the heat Q flowing to a low-temperature portion from the housing which is several times the amount of generated heat.
Severe conditions are described above such that the temperature of the laser diode is 0° C. and the ambient temperature is 70° C. , but in many cases, the temperature of the laser diode is lower than the temperature of the housing under actual operation environments of the laser module. Therefore, the peltier element consumes power for transferring the heat that is not required originally, to some degree or another.
In order to suppress such power consumption, a laser diode package to restrict the transfer of the heat from a housing to a peltier module is disclosed in the Japanese Patent provisional publication JPH05-235489. According to the laser diode package as disclosed in JP H05-235489, the xenon gas is filled inside the package that accommodates therein a laser diode (LD) mounted on a thermoelectric cooling module (TEC). The xenon gas is inert and low in thermal conductivity, and inhibits the transfer of the heat except that of the laser diode to the thermoelectric module.
As described above, as the method of inhibiting transfer of heat from the housing to the peltier module, there is a method of filling a space inside the housing with a gas of low thermal conductivity. Used as the gas of low thermal conductivity are inert gases such as xenon and krypton. In this case, only replacing the gas to be sealed in the module with such a gas of low thermal conductivity reduces power consumption of the peltier element, and enables an airtightness (i.e., hermetic sealing) test of the housing to be carried out using a mass spectrometer. Further, when the gas is filled at a pressure of the atmospheric pressure or more, the airtightness test of the housing is not severer in this method than in an evacuation method as described below. However, the area of a portion of low temperature is still large in the thermoelectric cooling module and the inflow of the heat from the housing is also large. Further, inert gases such as xenon and krypton are very expensive, and it is difficult to provide the laser module products at inexpensive prices.
As another method of inhibiting transfer of heat from the housing to the thermoelectric cooling module, there is a method of evacuating a space inside the housing. The transfer of heat due to heat conduction of gas is eliminated, and therefore, it is possible to more reliably reduce power consumption than in the former method. However, this method requires the housing to have extremely severe airtightness and little gas emission quantity. In other words, in order for the above-mentioned structure to exhibit vacuum insulation effect, the pressure inside the housing is approximately one millionth or less of the atmospheric pressure. The heat insulation characteristics deteriorate gradually as the pressure increases, and the vacuum insulation effect disappears when the pressure is about one hundredth of the atmospheric pressure. In other words, the state becomes the same as described above where the heat transfer exists from the housing to the thermoelectric cooling module. Generally, reliability of about 25 years is required of a laser module as a signal light source. In order to maintain the pressure inside the housing to keep vacuum insulation for these years, an allowable leak amount in the housing is less than 10−12 Torrl/s.
The leak amount is detectable by using a quadrupole mass spectrometer only in the case where probe gas can be supplied to the laser module at substantially the atmospheric pressure. However, when the probe gas is actually filled in the housing, the vacuum insulation deteriorates, while a small amount of probe gas makes detection by a quadrupole mass spectrometer difficult. The same airtightness is required in producing a vacuum tube, but it is possible to perform detection of airtightness because the vacuum tube has the same structure as that of an ionization vacuum gauge. In the laser module, since the module is not provided with such a structure, detection of leak is remarkably difficult operation.
Vacuum insulation deteriorates not only when gas flows into the housing from air but also when gas absorbed or occluded inside the housing invades the vacuum. Such a state is called gas emission herein. It is difficult to limit gas emission to a low level, unless latest considerations are given from examination of materials of the housing and components accommodated inside the housing to degassing due to heating. Thus, vacuum insulation is effective in power consumption of the laser module, but not actual resolution, because some technical breakthroughs are necessary to provide a laser module with high reliability, low power consumption and inexpensive price.